Research good news for coralline algae, but not necessarily for reefs

Author

PhD Student, Research School of Physics and Engineering, Australian National University

Disclosure statement

Merinda Nash does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

As Doha disappoints on delivering any real progress on reducing global CO2 emissions, new research demonstrates that a key component of coral reef structures may be more resilient in the face of increasing CO2 levels, and consequent declining seawater pH, than was previously thought.

However, while this is a good news story for the reefs, it does not mean that the entire reef is going to survive the negative impacts of ocean acidification associated with rising atmospheric CO2.

While investigating the mineral structure of a common coral reef alga, Porolithon onkodes, we found that there was an extra mineral, dolomite, present in many of the algae collected from the high-energy reef front environment.

These algae are known as coralline algae because, similarly to corals, they produce a carbonate skeleton. Corals produce a carbonate mineral CaCO3 called aragonite. The coralline algae form magnesium calcite, a mineral that is mostly CaCO3 but with 10-20% substitution by magnesium for calcium.

In many reefs the upper-most reef front structure is predominantly built by coralline algae, as delicate branching corals cannot develop in this wave break zone. Therefore, discovering that this algae looks to be a survivor under higher CO2 scenarios is most definitely good news, particularly for tropical island communities that are protected from high energy waves by these algal ridges.

The discovery that these algae produce dolomite, which is 50% magnesium instead of calcium and chemically very stable, was in itself an exciting discovery. Dolomite is most familiar to people as the mineral that gives the Dolomite Alps of Italy their name: dolomitized fossil coral reefs dominate the alps.

This dolomitization process was thought to be a chemical alteration of the reef limestone that took place long after a coral reef had died. Discovering that living algae in modern coral reefs can form this dolomite prolifically meant reconsidering what we thought we knew about the chemical stability of these algae.

Our experiments showed that dolomite corallines had 6-10 times less dissolution of the skeleton compared to the coralline algae without dolomite. Although dissolution increased in the high CO2 water, the total rate was still minimal. We found that dolomite was common in algae from many tropical reefs, but it seems to be restricted to the shallow, highest energy parts of the reef.

What does this finding mean for the Great Barrier Reef and other tropical reefs? First, I must make clear that our research related to coralline algae, not corals.

The Canberra Times and The Age ran the story with the headings “Coral may be climate change’s silver lining” and “Resistant algae good news for coral”. Considering our research was actually on algae, not coral, the Age has the most accurate headline. We are sadly accustomed to the critical role of coralline algae being overlooked in favour of the more visually appealing corals.

Coralline algae are not the same as the symbiotic zooxanthellae algae that live within the coral branch. Coralline algae are a pink encrusting algae that grow over dead and living corals and other reef substrate. The reef front below the exposed coralline algae surface is typically built of overlapping layers of coral and coralline algae. Our results demonstrate that the structural role provided by this coralline algae looks set to continue under higher CO2 levels than we previously thought.

The Australian reported our research with two other positive coral news stories with the headline “Forget the doom: coral reefs will bloom”. The coral stories related to corals thriving in conditions thought to be inhospitable to their survival.

On the face of it this all points to a positive future for the coral reefs. However, the key point that is often missed is that a coral and a coral reef are not the same thing. Just because a coral grows, it does not necessarily follow that a reef will form, just as a few trees growing in harsh conditions does not indicate a forest can form.

The recent CSIRO marine report card for ocean acidification identified the need for research at an ecosystem scale to understand the complex and sometimes inter-related responses of various reef organisms to ocean acidification.

It is possible to have a reef made of coralline algae without corals, as happened ~16 million years ago during the Mid-Miocene climactic optimum when CO2 and temperatures were higher than today. However, a reef made only of coralline algae will not support the biodiversity presently found on our tropical reefs.

With the Doha talks failing, maybe world leaders are counting on our world being more resilient to climate changes than we thought. While this research shows that coralline algae may be more resilient than we thought, unfortunately, we still can’t rely on this to save our reefs.